|Review of “Atmospheric forcing of sea ice anomalies in the Ross Sea Polynya region” by Dale et al. in The Cryosphere.|
Coastal polynyas in the sea ice around the Antarctic continent are known to be regions of major importance for the local ecology and for the global ocean overturning circulation. They occur on rather small scales and are subject to high temporal and spatial variability, making it difficult to analyze the driving processes in coarse resolution model, reanalysis, or satellite data. Dale et al. use data from an automatic weather station that is installed at the edge of the Ross Ice Shelf to study the effect of wind and temperature variations on sea ice variations in a subarea of the Ross Sea Polynya, one of the largest polynyas in Antarctica. They find that the opening of the polynya in this region is significantly correlated with high wind speed anomalies and that the closing is related to weak wind speed anomalies. A cross-correlation analysis shows a fast response of the polynya opening to high wind speeds and a slow response to weak winds and temperature anomalies. From this analysis, together with an analysis of sea-ice motion, the authors suggest that the fast observed opening of the polynya is driven an off-shore sea-ice advection and the slower closing is driven by thermodynamic sea-ice growth.
This study addresses an important and relevant issue and is in principal suitable for publication in The Cryosphere. While the methodology, analysis, and conclusions seem mostly robust, I have some concerns regarding the presentation and structure of the manuscript. I found it very difficult to read and to follow the line of thought, which should be improved alongside some other concerns listed below before I would recommend the publication of the manuscript. A general advice would be to organize and use paragraphs in a more structured sense, i.e., to have one topic per paragraph with the principal idea of the paragraph expressed in the first sentence and the concluding statement in the last sentence. Moreover, it would be easier to read if the paragraphs would neither be only 2-3 sentences long nor a full page. I think in some places (see below) the text could be a bit more focused and concise and certainly there are still quite some typographical errors that require a careful check (due to temporal constraints I cannot list them here, please check).
The editor asked specifically whether I thought that the concerns by Referees #1 and #2 were sufficiently addressed. I think the authors did address these issues sufficiently in most cases. I listed some remaining issues and some additional concerns that should be addressed in a revision below.
- Large parts of the introduction deal with the increase of Antarctic sea ice over recent decades, but I do not really see how this study contributes to this question. It might well be that polynya production and export in the Ross Sea changed in concert with the observed ice expansion in the Ross Sea (e.g. Drucker et al., 2011 and Haumann et al., 2016), but the causal link between the ice edge variation and the polynya variation is less well established. This study does neither study variations of the sea ice extent nor long-term trends. So, I would recommend to focus the on the more important aspects, which are local aspects and temporal variability. One of the aspects is e.g. the effect on deep water formation (e.g. Ohshima et al. 2013) or ecology.
- The introduction could be better structured. Right now the reader is being put back and forth between sea-ice trends and polynya processes. It might be helpful to point out more clearly the gap of knowledge, which is the effect of the local wind system on the polynya processes that cannot be resolved by models and end on the contribution of this study.
Data and methods:
- It is not fully clear to me which years are used in the end for the analysis. The authors list a number of different time periods over which the different data sets are available, but ultimately I guess the analysis period is constrained by the availability of the AWS data, which only existed after 2000. So, all data sets, analysis, figures should consistently build on this period and has to be mentioned somewhere.
- Was the data de-trended and de-seasonalized before performing the analysis? I think it would change the results much, but it should be done when using a correlation analysis.
- The drift vectors are calculated over multiple grid boxes that have a size of 25 km and one vector is obtained from differencing multiple grid boxes. Therefore, a resolution of 25 km for the final motion product seems not adequate as the actual resolution is coarser than that. Ideal would be to provide the vectors at their actual resolution or describe this caveat.
- Throughout the manuscript the authors use sea-ice concentration (SIC) in the text but Figure 2 actually shows sea-ice area while the text says it would show SIC. It needs to be clarified what quantity is used when and why.
- Lines 4.24-25: Please describe how you actually calculated the ice velocity from the cross-correlation field or provide a reference.
- The splitting of the results section as suggested by one of the earlier reviewers definitely helped to improve the readability. However, some more structuring would be helpful. The first paragraph of section 3.3 should be split and restructured. I honestly have difficulties to follow and I think that some parts might be cut. For example, I found the explanation of lines 6.3-6 regarding the wind stress more confusing than helpful at this point.
- Section 3.4 is supposed to be about ERA-Interim data (title) but in fact half of it is about the AWS data, which I found confusing. Please restructure to resolve this issue. Also I think the information in this section could be more confined and the second paragraph should be shortened, split or restructured for clarity.
- I do not understand Figure 4 a. It shows a negative correlation between the wind speed and SIC at low wind speeds. However, figure 5, the text, abstract and conclusions say that at low wind speeds the ice concentration is higher. Therefore, I would expect a positive correlation here. To me this seems inconsistent. Could you explain?
- Lines 10.33-11.1: To me this formulation does not make sense. How could sea-ice drift form ice. It could only advect ice into this region, which would be the reversal of the opening process. However, this seems unlikely. On the other hand, the statement that “wind-driven processes” are not responsible for the recovery, seems unsupported to me as well. As there would be no recovery phase without the prior wind-driven off-shore advection.
Currently many of the labels, legends, and arrows are rather small. Please enlarge them for readability.
Some further suggestions:
Lines 1.2-3: Delete or reformulate “and possible consequences for sea ice production”. Sea ice production is not “investigated” in this study. I agree that the study has implications for the sea ice production but there is no related analysis.
Lines 1.18-19: “This suggests …”. I think that this sentence is slightly problematic, since the thermodynamic ice production mostly occurs because the ice was dynamically removed previously. So, it is not a pure thermodynamic process that leads to the polynya ice production. I agree that the ultimate new ice formation occurs due to thermodynamics, but isn’t that the case by definition? Or in other words how could new ice be produced dynamically. Please reformulate.
Line 1.22: “growth” -> “expansion”
Lines 2.20-23: I do not see the link between the ice extent or these processes to this study or what was discussed in this paragraph.
Line 2.25: “… wind stress.” This statement requires in my view a reference.
Lines 2.29-31: SIE is not used in this study, please reformulate or remove.
Lines 3.7-17: Please be more precise on what you actually do in this study and remove or replace the rest for helping the reader what to expect from this study. Please take out the ice production, as this study does not analyze ice production.
Line 3.22: I think the record starts in “1978”. Anyway, please only indicate the time period that you actually used and remove the information on the rest of the record.
Lines 4.5-10: This seems to belong rather to the results section than Data and Methods.
Line 5.23: “Scale factor” -> Do you mean “slope of the regression line”?
Lines 6.14-15: Please be more specific on “dynamic” or “thermodynamic process”, i.e. “northward advection”, “ice formation”, etc..
Lines 6.24-28: As the sea ice is divergent in this situation, I doubt that ridging and rafting processes are responsible for the delay. I would think that it is rather the ice internal stress that might cause the delay in the ice response.
Lines 7.8-11: I do not understand why there would be a warm anomaly if the southerly winds increase. I would expect the opposite, i.e. a cold anomaly as the air coming from the ice shelf is much colder.
Lines 10.24-25: The 10 m wind speed in ERA-Interim should probably be given in the Methods section as well. More importantly here you state the AWS measures wind speed at a height of 2-3 m, which is inconsistent with lines 3.23, where you state that it measures wind speed at 10 m. Please correct either value and give a precise height.
Line 13.11: SIC -> sea ice
Line 13.14: “reanalysis” -> “coarse resolution atmospheric reanalysis data”
Figure 1: Specify that grey lines show topography. Please add reference to source of topography. Also here the standard deviation should be calculated from the de-trended and de-seasonalized fields and only the period that is actually analyzed in the paper should be used for consistency. Please add that the red box is also the area used for Figure 2. Please add a legend for the wind rose.
Figure 2: delete “dotted” for me these look like solid lines. “grey” -> “black”
Figure 5 and 7: I guess these are composites. Please indicate this and the time period (years) over which the composite is formed.